organicchemist25
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Reduced pressure distillation question and smoke?
So, I went onto distilling an essential oil for the first time. Oil of wintergreen. It BP is 220 C and my current vacuum, while my chemglass is in
the shop for overheating, looks to be pulling about 100 torr. Not all that good, but its what im working with at the moment. Everything seemed to be
sealed up nicely at all the joints, and from the nomograph in zubricks manual it should have been coming over around 140ish C.
I started to get something around 100C and just kept climbing to 110C then 120 C and then to 13o C and 140C and going up to 150ish C. All the while,
I had distillate coming over. I did not have a pause in-between temp ranges. I figured all the early temps were foreruns, but it just kept climbing
and carrying over distillate.
So, I cut it off. Then I cut the hotplate off and removed it from the oil and then a couple minutes later cut the vacuum off.
I got a light, white smoke or steam in the boiling flask right after I cut off the vac. Ive never seen that before...any explanation?
I feel it was sealed up nicely. It held the vacuum constant after I turned the vac off before starting up the distillation confirming I had tightly
greased seals. The gauge did not budge after 5-10 minutes after pump was cut off.
Should my temp constantly rise like 40 degree C and dropping distillate the whole time as its climbing? That would be new to me, but I am aways
learning something new. It was on a slow drip, so I wasn't cooking the heck out of it. It was like a drop every 1-2 seconds.
Thanks
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Sulaiman
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Guessing only;
. with the column insulated there will be little refluxing hence fractionating occuring,
so the setup is basically a simple distillation
. if the pressure is fluctuating/pulsing then the b.p. will fluctuate as a result,
i.e. if there is a range of pressure there will be a range of b.p.
CAUTION : Hobby Chemist, not Professional or even Amateur
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organicchemist25
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The pressure remained dead on constant the entire time the only reason I ended up insulating was due to the top half of the flask not submerged and
vigreux column wouldn't begin to have anything come up the column. It was still cool to the touch after the peanut oil was 170+ for over and hour and
I had the hotplate gradually increasing and ended up at 550 C and that's where I was still unable to get it flow up the column. I took temp of the
hotplate and it was accurately putting off that much heat. It's new, and a Corning model. After I insulated it it started to come over after about
10-15 minutes. I couldn't get the vapors to travel up the column prior to insulating. I figured and seemed pretty apparent that the column was too
cool.
I had read a few different times where someone talked about insulating the column.
Should I only insulate the the top of the flask and maybe a quarter of the lower half of the column?
What else could I do if I was doing that wrong?
I appreciate your reply.
[Edited on 30-10-2017 by organicchemist25]
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Dr.Bob
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You should allow the apparatus to cool before releasing the vacuum. The smoke was possibility from the oil vapors reacting with the air that leaks
in once the vacuum is turned off or released. In the case of air reactive materials, this can start a fire in the rbf or condenser. So best to
turn off the heat, let the apparatus cool some under vacuum, then bleed inert gas in, if possible, as the vacuum is released, or at least make sure
that the flask has cooled. A friend had a co-worker injured when they opened a large distillation from vacuum to air while very hot, and the
material in side violently reacted. Not a common problem, but one to be aware of. This is especially true of many essential oils, flavors, and any
material with aldehydes, double bonds, or other oxidizable materials.
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Sulaiman
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I am in no way defending my guess, just offering ideas;
your bourdon gauge may not indicate pulsing pressure variations due to its time constant,
since its only a guess, one way that I can imagine testing for this potential problem could be;
add a funnel with the large end covered in Al or PE kitchen foil/film to your vacuum system
(small end of funnel connects to vacuum line)
pulsing at audio frequencies will make the funnel act like a loudspeaker, so pulsing can be heard.
pulsing at lower frequencies can be checked for by touching the foil or film and feeling for vibrations.
...................................................
Without adding a reflux condenser, and using a packed (Hempel) column or similar complications.
I guess that the best fractionation will be achieved by your setup when the boiling rate is maximum
(provided that there is stirring there should be no hot spots)
and the insulation is adjusted to give just enough reflux in the column to allow some product to come over.
As this is exactly what you seem to have done, I can't think of any improvement. 
................................................
One other possibility - a miscible impurity, possibly forming an azeotrope ?
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organicchemist25
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Quote: Originally posted by Dr.Bob  | | You should allow the apparatus to cool before releasing the vacuum. The smoke was possibility from the oil vapors reacting with the air that leaks
in once the vacuum is turned off or released. In the case of air reactive materials, this can start a fire in the rbf or condenser. So best to
turn off the heat, let the apparatus cool some under vacuum, then bleed inert gas in, if possible, as the vacuum is released, or at least make sure
that the flask has cooled. A friend had a co-worker injured when they opened a large distillation from vacuum to air while very hot, and the
material in side violently reacted. Not a common problem, but one to be aware of. This is especially true of many essential oils, flavors, and any
material with aldehydes, double bonds, or other oxidizable materials. |
right on, Thank you for that important tip!!
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organicchemist25
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| Quote: Originally posted by Sulaiman | I am in no way defending my guess, just offering ideas;
your bourdon gauge may not indicate pulsing pressure variations due to its time constant,
since its only a guess, one way that I can imagine testing for this potential problem could be;
add a funnel with the large end covered in Al or PE kitchen foil/film to your vacuum system
(small end of funnel connects to vacuum line)
pulsing at audio frequencies will make the funnel act like a loudspeaker, so pulsing can be heard.
pulsing at lower frequencies can be checked for by touching the foil or film and feeling for vibrations.
...................................................
Without adding a reflux condenser, and using a packed (Hempel) column or similar complications.
I guess that the best fractionation will be achieved by your setup when the boiling rate is maximum
(provided that there is stirring there should be no hot spots)
and the insulation is adjusted to give just enough reflux in the column to allow some product to come over.
As this is exactly what you seem to have done, I can't think of any improvement.
................................................
One other possibility - a miscible impurity, possibly forming an azeotrope ? |
I will use the Al and funnel next time I do another run. This weekend will probably be a good time. It did have a stir bar turning it was at 130 rpm
anything higher and it would break free of the magnetic attraction.
Thank you!
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DJF90
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What was the oil bath temperature? Plate surface temp means very little - it might be set to 550*C but when its got a system to heat up then the
wattage (energy input) probably isn't up to the job. I've had similar issues before with a hotplate not being able to heat a (smallish) sand bath and
distillation flask to 200*C. In this case making a foil "teepee" around the flask works much better as there is much less thermal mass to heat in this
case.
I'm not sure the Al foil + funnel idea that Sulaiman has suggested would be any good for reduced pressure as I can imagine difficulties with
maintaining a sealed (vacuum-tight) system...
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Sulaiman
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I just had a terrible thought of the foil failing under maximum vacuum
I don't have (and don't want) experience of sudden catastrophic vacuum failure with hot glassware and reactants,
but I bet its not funny,
so maybe try with just the pump, trap (if any) and funnel ?
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organicchemist25
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| Quote: Originally posted by Sulaiman | I just had a terrible thought of the foil failing under maximum vacuum
I don't have (and don't want) experience of sudden catastrophic vacuum failure with hot glassware and reactants,
but I bet its not funny,
so maybe try with just the pump, trap (if any) and funnel ? |
I definitely have a couple nice dry ice traps. I will just try the foil with flask and trap. ill update on that when i do it. I may do it tomorrow
evening. Im curious to know.
Also, Ive had a new Mcleod manometer for about a year or so and just finally ordered some triple distilled mercury to fill it with. Would that signal
any fluctuation in vacuum pressure? I should have the Hg by end of the week.
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Sulaiman
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I just tried Al foil over a 100mm dia glass funnel attached by a short length of tubing to my little piston vacuum pump,
usually this pump gives -75 to -80 kPag, 20 to 25 kPa (150 to 190 mm Hg) absolute pressure.
The foil held up ok,
I could not hear or feel any diaphragm/foil vibrations other than those that I attributed to the pump,
so it is certainly not a sensitive test 
EDIT: Now that I think about it, I will not be surprised if you have a negative result,
even if the pressure fluctuates by say +/- 10%, or even multiplied or divided by two,
the absolute pressure change will be negligible compared to atmospheric pressure,
so very little diaphragm movement is expected.
(seems like just one more of my dumb 'brilliant' ideas 
[Edited on 1-11-2017 by Sulaiman]
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